1. Field of the Invention
This invention relates to electro-optic (EO) sensors configured to detect both broadband and multi-filtered signatures. These EO sensors may be used in guided munitions and surveillance systems among other applications.
2. Description of the Related Art
Many guided munitions (e.g. self-propelled missiles, rockets, gun-launched projectiles or aerial bombs) use an EO sensor to detect and guide the munition to its target. The sensor's ability to detect and initiate target tracking at long standoff ranges to the target is critical. To accomplish this, the sensor must maintain a minimum signal-to-noise ratio (SNR) from the target signature.
In a semi-active laser (SAL) mode, the sensor detects active guidance radiation in the form of laser radiation from a SAL designator that is reflected off of the target and locks onto the laser spot to provide line-of-sight (LOS) error estimates at an update rate required by the guidance system. The standard SAL laser designator produces laser radiation at 1.064 microns in the Near IR. The optics spatially encode an angle of incidence of the SAL laser radiation (e.g. a laser spot) at an entrance pupil onto the SAL, detector. A quad-cell photodiode provides sufficient resolution to determine the LOS error estimate.
In a passive imaging mode, the sensor detects IR radiation emitted from or reflected off of the target. The sources of IR energy are not artificial; they typically follow the laws of Planck radiation. The source may be the blackbody radiation emitted by the target directly or may, for example, be sunlight that is reflected off of the target. The passive imaging radiation from a typical target is at long range such that the electromagnetic wavefront at the sensor is considered to be composed of planar wavefronts. The structure of the target is imprinted on the composite wavefront as a summation of planar wavefronts with different slopes. The optics convert these slopes to spatial offsets in the image plane to form an image of the target on the pixelated IR imaging detector.
The IR imaging detector typically operates in the Short-Wave Infrared (SWIR) (1-2.5 um), Mid-Wave Infrared (MWIR) (3-5 um), or Long-Wave Infrared (LWIR) (8-14 um) electromagnetic radiation bands. With currently available technologies such as opto-mechanical scanning, staring focal plane array (FPA) or digital scanning (known as a “Rice pixel”), this detector may exhibit an effective spatial resolution, for example, of anywhere from 32×32 to 4,000×3,000 pixels. Selection of the desired band(s) for the passive imaging sensor depends on the target of interest and the expected atmospheric absorption bands. The SWIR Band is typically used in night conditions to provide high contrast. The MWIR band is selected if the expected targets are relatively hot (e.g. planes, missiles, etc.). The LWIR band is typically used to image targets that have operating temperatures slightly above the standard 300K background.
Some guided munitions use a “dual-mode” EO sensor that includes both a SAL mode and a passive imaging mode. The dual-mode EO sensor comprises a primary optical element having a common aperture for collecting and focusing SAL laser radiation and passive imaging radiation. A secondary optical element separates the SAL laser and passive imaging radiation by spectral band and directs the SAL laser radiation to a SAL detector and directs the passive imaging radiation to an IR imaging detector. The SAL mode is typically used early in flight to detect the target at a maximum standoff range. The passive imaging mode is typically used mid-flight or at the end of flight to process a more highly resolved image to determine whether or not the target is of interest or to choose a particular aimpoint on the target. The passive imaging mode operates at a much higher spatial resolution than the SAL mode. The passive imaging mode may be used to provide LOS error estimates to track the target when SAL designation is not available. However, due to its much higher spatial resolution, and detection of passive radiation, there are fewer incident photons per pixel, and the passive imaging mode may not have the sensitivity (i.e. SNR) to acquire and track the target at long ranges at the desired update rate.
Surveillance systems use EO to detect, identify and track targets of interest. These systems typically operate at shorter ranges to the target than the initial acquisition ranges associated with guided munitions and have enough physical space to include a larger aperture, and thus it is much easier to acquire and maintain a target signature with adequate SNR. However, surveillance systems may require more finely resolved target signatures to differentiate and identify targets. To accomplish this, the EO sensor may filter the incident radiation into multiple measurement bands to increase the dimensionality or diversity of the target signature. For example, the EO sensor may filter the radiation into multiple narrow or broad spectral bands. Alternately, the EO sensor may filter the radiation into its four polarization states to provide the Stokes vectors to determine the Degree of Linear Polarization, which is very useful in discriminating between natural and man-made targets. In one approach, the EO sensor cascades filtered beam splitters to split the incident radiation into the desired number of filtered sub-channels and directs the filtered light in each sub-channel to a different detector. The light may be filtered according to wavelength or polarization. The attenuation of the incident radiation incident on each detector, hence the reduction in SNR is considerable but tolerable in typical surveillance systems. In another approach, the EO sensor includes single pixel filters that are positioned pixel-by-pixel on the detector to sample the field-of-view (FOV). This approach reduces the amount of attenuation and shares a common detector but is time-consuming, costly and fragile.